1. Field of the Invention
The present invention relates to a carrier reproducing apparatus and method used in an for demodulating for a digitally modulated signal used in satellite communication, satellite broadcasting, etc.
2. Description of the Related Art
Satellite communication networks are configured based on so-called multiple access, that is, use of one or more satellite-mounted repeater for setting communication paths simultaneously among a plurality of ground stations.
There are a variety of methods for how to divide communication paths for multiple access. Frequency division multiple access (FDMA) is one of them.
As one type of the FDMA, there is the single channel per carrier (SCPC) system wherein each communication path is comprised of one line.
In SCPC, each carrier is sent to a satellite modulated by a single channel. In the past, FM modulation, digital PSK modulation, etc. have been used. SCPC is advantageous in the points that requests can be easily assigned and use may be made of voice activation, i.e., setting a threshold value at the voice level, considering voice communication to not be performed while the threshold value is not exceeded, and cutting a sending carrier instantaneously at every such instant. This enables an improvement in the, efficiency of use of the relays. SCPC is very efficient in terms of system efficiency and flexibility in a network having a large number of low-traffic ground stations.
However, since a large number of carriers are commonly amplified and the symbol rate is a low several M symbols per second (MSPS), there is the disadvantage of a large effect of the intermodulation product and difficulty of carrier pull-in at the time of demodulation.
As one technique for solving such a problem, for example, there is the demodulation circuit for a digital modulation wave disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-120992.
In this demodulation circuit, the carrier is reproduced using two feedback loops. Specifically, an internal feedback loop (Costas loop) performs frequency pull-in and phase synchronization based on a phase of the received signal. Also, an external feedback loop detects frequency error in the internal frequency loop and continuously controls the system to suppress that frequency error and performs frequency pull-in and phase synchronization discontinuously using an offset value when a locked state of the internal feedback processing is released. As a result, it is possible to shorten the time from when the locked state of the internal feedback processing is released until when it becomes locked next.
Summarizing the problem to be solved by the invention, since the demodulation circuit for a digital demodulation wave disclosed in the above Japanese Unexamined Patent Publication (Kokai) No. 6-120992 detects frequency error in the internal feedback loop, there is the disadvantage that the operation becomes unstable at the time of a low S/N.
Also, since external feedback processing is always performed, there is the disadvantage that the frequency pull-in and phase synchronization become unstable in relation with the internal feedback processing.
Furthermore, it is necessary to operate a roll-off filter at a high frequency the same as an A/D conversion circuit before the external feedback loop (frequency of at least twice symbol rate), so the roll-off filter becomes complex and large in size.
An object of the present invention is to provide a carrier reproducing apparatus and method enabling stable operation at the time of a low S/N.
Another object of the present invention is to provide a carrier reproducing apparatus and method enabling a simple and small configuration.
To attain the above objects, according to a first aspect of the present invention, there is provided a carrier reproducing apparatus for reproducing the carrier of a received signal, comprising a first multiplying circuit for multiplying the received signal with a first feedback signal; a filter circuit for filtering a signal output from the first multiplying circuit to remove a signal component having a frequency adjacent to carrier frequency; a second multiplying circuit for multiplying the filtered signal with a second feedback signal; a phase detection circuit for detecting a phase of a signal output from said second multiplying circuit and generating a phase signal; a first numerical control circuit for generating said second feedback signal having an oscillation frequency in accordance with said phase signal; a lock detection circuit for detecting a locked state of a phase of the signal output from said second multiplying circuit based on said phase signal; a second numerical control oscillation circuit for generating said first feedback signal having an oscillation frequency in accordance with a third signal; and a tracking circuit for controlling said third signal so that said phase signal does not exceed a predetermined value when a phase of the signal output from said second multiplying circuit is locked and the phase signal exceeds said predetermined value.
The mode of operation of the carrier reproducing apparatus according to the first aspect of the present invention are as follows.
A first feedback loop is comprised of the second multiplying circuit, phase detection circuit, and first numerical control circuit.
A second feedback group is comprised by the first multiplying circuit, filter circuit, tracking circuit, and second numerical control circuit.
In the lock detection circuit, a locked state of a phase of the signal output from the second multiplying circuit, that is, a locked state of the first feedback loop, is detected based on a phase signal generated by the phase detection circuit.
In the tracking circuit, a third signal input to the second numerical control circuit is controlled so that the phase signal does not exceed a predetermined value when a phase of the signal output from the multiplying circuit is locked and the phase signal exceeds the predetermined value.
As explained above, in the carrier reproducing apparatus of the first aspect of the present invention, since the third signal input to the second numerical control circuit is controlled when the first feedback loop is locked, frequency pull-in and phase synchronization in the first feedback loop can be stabilized.
Preferably, the apparatus further comprises a loop filter circuit for filtering out a high-band component of said phase signal; said first numerical control circuit generates said second feedback signal having an oscillation frequency in accordance with the phase signal from which the high-band component is removed; and said tracking circuit, when a phase of the signal output from said second multiplying circuit is locked and the phase signal wherein the high-band component is removed exceeds a predetermined value, controls said third signal so that the phase signal from which the high-band component was removed does not exceed said predetermined value.
Preferably, said tracking circuit increments or decrements a value indicated by said third signal.
Alternatively, preferably the apparatus further comprises an offset setting circuit for generating said third signal by using a predetermined offset value when a phase of the signal output from said second multiplying circuit becomes unlocked.
Preferably, when a phase of the signal output from said second multiplying circuit becomes unlocked due to an interruption of said received signal or an operation for selecting a station, said tracking circuit holds a control state of said third signal immediately before unlocking and controls said third signal based on said held control state when said received signal is recovered or said operation for selecting a station is completed.
Alternatively, when a phase of the signal output from said second multiplying circuit becomes unlocked to an interruption of said received signal or an operation for selecting a station, said offset setting circuit holds said offset value used immediately before the unlocked state and controls said third signal using said held offset value when said received signal is recovered or said operation for selecting a station is completed.
Alternatively, preferably said apparatus further comprises an A/D conversion circuit for converting said received signal from an analog to digital format, and said first multiplying circuit multiplies said received signal after said A/D conversion with said first feedback signal.
According to a second aspect of the present invention, there is provided a carrier reproducing apparatus for reproducing a carrier of a modulated received signal, comprising an A/D conversion circuit for sampling said received signal at a frequency of at least n (n greater than 2) times a symbol rate of said modulation; a first filter circuit for filtering said sampled received signal to remove a signal component having a frequency adjacent to carrier frequency; a down sampling circuit for thinning said filtered signal; an interpolation circuit for interpolation by said thinned signal to generate a signal having a frequency of n times said symbol rate; a multiplying circuit for multiplying a signal generated by said interpolating circuit with a feedback signal; a phase detection circuit for detecting a phase of a signal output from said multiplying circuit and generating a phase signal; a second filter for removing a high-band component of said phase signal; and a numerical control circuit for generating said feedback signal having the oscillation frequency in accordance with said phase signal from which the high-band component was removed.
The mode of operation of the carrier reproducing apparatus according to the second aspect of the present invention is as follows.
The A/D conversion circuit samples the received signal at a frequency of at least n (n greater than 2) times the symbol rate of the modulation.
Next, the first filter circuit filters out adjacent carriers from the sampled received signal.
Then, the down sampling circuit thins the filtered signal.
Then, the interpolation circuit interpolates by the thinned signal to generate a signal having a frequency of n times the symbol rate,
The generated signal is processed in the feedback group.
In the feedback group, the multiplying circuit multiplies the signal with a feedback signal, the phase detection circuit detects a phase of the signal output from the multiplying circuit and generates a phase signal, the second filter circuit filters out the high-band component of the phase signal, and the numerical control circuit generates the feedback signal having an oscillation frequency corresponding to the phase signal from which the high band component was filtered out.
According to the carrier reproducing apparatus of the second aspect of the present invention, the operating frequency of the second filter circuit can be made lower than the operating frequency of the A/D conversion circuit and the second filter circuit can be made simple and small in size.
Preferably, the apparatus further comprises a second multiplying circuit for multiplying said sampled received signal with a second feedback signal when assuming said feedback signal is a first feedback signal and said multiplying circuit is a first multiplying circuit; a lock detection circuit for detecting a locked state of the signal output from said second multiplying circuit based on said phase signal; a second numerical control oscillation circuit for generating said second feedback signal having an oscillation frequency in accordance with a third signal; and a tracking circuit for controlling said third signal so that the output from said second filter circuit does not exceed a predetermined value when a phase of a signal output from said first multiplying circuit is locked and the output of said second filter circuit exceeds said predetermined value.
According to a third aspect of the present invention, there is provided a carrier reproducing method for reproducing a carrier of a received signal, including the steps of multiplying the received signal with a first feedback signal; filtering a signal output from said first multiplication to remove a signal component having a frequency adjacent to carrier frequency; multiplying said filtered signal with a second feedback signal; detecting a phase of a signal output from said second multiplication and generating a phase signal; generating said second feedback signal having an oscillation frequency in accordance with said phase signal; detecting a locked state of a phase of the signal obtained by said second multiplication based on said phase signal; generating said first feedback signal having an oscillation frequency in accordance with a third signal; and controlling said third signal so that said phase signal does not exceed a predetermined value when a phase of the signal obtained by said second multiplication is locked and the phase signal exceeds said predetermined value.
According to a fourth aspect of the present invention, there is provided a carrier reproducing method for reproducing a carrier of a modulated received signal, including the steps of sampling said received signal at a frequency of at least n (n greater than 2) times a symbol rate of said modulation; filtering said sampled received signal; thinning said filtered signal to remove a signal component having a frequency adjacent to carrier frequency; interpolating using said thinned signal to generate a signal having a sampling frequency of 2 times said symbol rate; multiplying said interpolated signal and a feedback signal; detecting a phase of a signal obtained by said multiplication and generating a phase signal; removing a high-band component of said phase signal; and generating said feedback signal having an oscillation frequency in accordance with said phase signal from which the high-band component was removed.